The invention relates to a device for injecting a fuel at a variable injection pressure, an example being a cam-driven pump-line-nozzle system. Such devices are used in direct injection systems in internal combustion engines.
In devices for injecting fuel on the order of the pump-line-nozzle system, the injection pressure is dependent on the driving rpm, or in otherwords the engine rpm. In such devices, only the injection onset can be controlled by a valve, acting as a magnetic switching valve; the pressure of the injection pressure is dependent on the driving rpm. Thus in this injection configuration, the pressure of the injection event cannot be preselected freely.
From U.S. Pat. No. 5,628,293, an electronically controlled fluid injector is known, with a fluid collection chamber and with a directly triggerable control element for opening the connecting line between the fluid collection chamber and the injection nozzle that protrudes into the combustion chamber of an internal combustion engine. In addition to the first, directly triggerable injection element, another pressure control element can be moved back and forth between two control positions. By means of the two switchable pressure control elements, hydraulic forces that act counter to one another can be balanced out. In this configuration, control of the pressure elements is done via two units, which are only partly secured against overpressure or an excess quantity in the event of failure of the control system.
With the proposal according to the invention of a device for injecting a fluid at variable injection pressure, the level of the injection pressure is independent of the engine rpm. The course of injection can be controlled as needed independently of the engine rpm, since the triggering of the control element, which is acted upon on its respective face ends by two spring means, is done electronically via a control unit. The onset of injection can likewise be defined and determined with extreme accuracy by means of a triggerable switching element. The course of injection of the single-cylinder injection pump with variable injection pressure is varied by the course of the piston motion toward top dead center. This variation can be defined by suitable shaping of the cam in the process of designing it. The actuating element, which is in the form of a roller rotatably supported on a piston rod, is for instance moved by a cam, in accordance with the contour of the cam. Accordingly, the course of the injection event can thereby be varied.
The proposed embodiment of a device for injection makes a major contribution to system safety, since filling of the pump chamber does not occur if the switching valve, preferably embodied as a fast-switching magnet valve, is without electrical current. The spring means on the side toward the switching valve generates a greater force and causes the control element to be pressed against the seat face and causes closure of the high-pressure-side inlet. This prevents filling of the pump chamber, and the system is incapable of injecting any fuel. If the control element, in the event of a malfunction, remains stuck in an open position, then a short circuit of the flowing fuel takes place from the pressure chamber into the low-pressure chamber. As a result, excess fuel can be prevented from achieving injection and causing engine damage.
By equipping the control element with a pressure stage in the region of the inlet-side bore for supplying the injection nozzle with fuel, the control element in interaction with the magnet valve can function as a safety valve. If a maximum possible system pressure is exceeded, an uncovering of the control edge in the low-pressure region takes place; that is, the inlet to the low-pressure chamber is uncovered on one face end of the control element. The fuel then flows directly from the pressure chamber into the low-pressure chamber, so that the forces occurring at the roller tappet do not exceed its load limits.
In the method according to the invention for controlling a device for injecting fuel, the pressure buildup in a single-cylinder pump unit takes place as a function of the stroke of the pump piston; this stroke is imposed by the camshaft via the actuating device received in the lower region of the pump piston. The course of injection can be controlled by suitable shaping of the cam. The end of pumping is brought about when the control element reaches an intermediate position at half the stroke length, and in this position, as a result of the mutually balancing forces of the spring means and of the magnet valve, it remains open on both seat faces toward the high-pressure-side injection nozzle inlet and at the outlet into the low-pressure chamber; the pressure thus drops rapidly. Injection at the nozzle is now suppressed.